Methods for detecting and differentiating plaque in vessel structures of patients with the aid of absorption computed tomography, and absorption CT systems used therefor are generally known in principle. For the most part, in this case contrast media are applied so as to display the vessel structures, and the presence of plaque is determined by more thoroughly viewing the edge region of the displayed vessels with absorption data. There are also attempts at determining the different consistency of plaque, calcified or soft, with the aid of absorption CT. Calcified plaque generally does not constitute a health risk, while so called soft plaque conceals a greatly increased risk of cardiac infarction. However, in determining and in particular in classifying plaque the problem arises that the absorption differences of different plaque types turn out to be relatively slight.
Two effects can be considered in principle for imaging by way of ionizing beams, in particular x-ray beams which occur when the radiation penetrates matter and are, specifically the absorption and the phase shift of the radiation penetrating an examination object.
Such phase contrast radiography or phase contrast tomography requires the phase shift caused by the object to be evaluated. By analogy with conventional absorption contrast x-ray radiography or absorption contrast x-ray tomography, it is possible both to prepare projected images of the phase shift, and to calculate tomographic displays of the phase shift from a multiplicity of projective images.
Such phase shifts can be determined by the use of interferometric gratings, and be used to produce projective or tomographic images. Reference may be made to the previously cited documents as regards this interferometric measuring method. In the case of this method, an examination object is transradiated by a coherent or quasi-coherent x-radiation and is subsequently guided through a phase grating of a period adapted to the wavelengths of the radiation, as a result of which there is firstly beam splitting, and there results from the position of the divided beams an interference pattern that is modulated by the phase shift modulating from the object. This interference pattern is measured by means of a subsequent analyzer detector arrangement such that the phase shift can be determined for each “beam” or each detector element that generates an image pixel. This can be performed either by a number of measurements with offset analyzer gratings or by direct determination of the phase shift with the aid of a detector element that is subdivided at least in a threefold fashion perpendicular to the line orientation of the grating lines of the phase grating.
With regard to the refractive index, which is given for x-ray beams byn=1−δ−iβ, the absorption is a function of the magnitude of the imaginary decrement with β, which is related to the mass absorption coefficient μ/ρμ/ρ=4πβ/λ,λ being the wavelength, μ being the linear absorption coefficient, and ρ being the mass density.
The phase shift follows from the real part of the refractive index 1−δ. The phase shift Δ of an x-ray wave in matter is given, in comparison with a vacuum, byΔ=2πδT/λ, T being the thickness of the matter and δ the real decrement of the refractive index.
Phase contrast tomography requires evaluation of the phase shift caused by the object. By analogy with the absorption CT, it is possible to calculate from projection data a three-dimensional data record that shows the spatial distribution of the real part of the refractive index 1−δ.
Since the phase of a wave cannot be measured directly, the first requirement is to convert the phase shift into a measurable intensity by way of the interference between the wave to be examined and a reference wave. Carrying out such measurements in practice, both with reference to projective images and with reference to tomographic images is, for example, in European patent application EP 1 447 046 A1 and in the German patent applications with the file references 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 291.4, 10 2006 015 356.1 and 10 2006 015 355.3.
In the context of the patent application, reference is often to be made to the following state of affairs relating to the problems of “coherent x-radiation”, “coherent x-radiation sources” and “quasi-coherent x-radiation sources”:
The emission of x-ray photons via laboratory x-ray sources (x-ray tubes, secondary targets, plasma sources, radioactive sources, parametric x-ray sources, channeling radiation), and also of conventional synchrotron radiation sources of first to third generation is subject to stochastic processes. The emitted x-radiation therefore has no special coherence per se. The radiation of x-ray sources behaves, like spatially coherent radiation in phase contrast radiography and tomography when the viewing angle at which the source appears to the viewer or the object, the grating or the detector is sufficiently small. The so called lateral coherence length L:
  L  =      λ    ⁢                  ⁢                  a        s            .      may be specified as a measure of the spatial coherence of an extended x-ray source. Here, λ is the wavelength, s the transverse source size, and a the source/viewer distance. Some authors also designate half the above defined value as spatial coherence length. The exact value is secondary; what is important is that the coherence length L be large by comparison with the (lateral) dimensions of the spatial region from which beams are to interfere with one another.
For the purpose of the patent application, coherent radiation is understood to be a radiation that leads to the formation of an interference pattern under the given geometries and distances from the desired x-ray/optical grating. It is self evident that the spatial coherence, and thus the spatial coherence length, are always determined by the triplet of variables comprising wavelength, source size and viewing distances. For the purpose of a compact formulation, this state of affairs has been shortened to expressions such as “coherent x-radiation” “coherent x-radiation source” or “point source for generating a coherent x-radiation”. These abbreviations are based on the fact that the wavelength of the energy E of the x-radiation in the application discussed here is limited by the desired penetrating power of the examination object, on the one hand, and the spectrum available in the case of laboratory x-ray sources. The distance a between source and observer is also subject to certain restrictions in medical diagnostics. The last degree of freedom therefore usually remains the source size s, even when the relationships between source size and tube power set narrow limits here.
The source grating permits the use of greater and thus more powerful x-ray sources.
The narrow slits of the source grating ensure observance of the required spatial coherence of all the beams that emerge from one and the same slot. Only photons from one slot can interfere with one another, that is to say they superpose with correct phase. Although no correctly phased superposition is possible between the photons from slit to slit of the source grating, with suitable tuning of the source grating period d0 and the interference pattern period d2 as well as the distance 1 between the source grating G0 and phase grating G1, and the distance d between the phase grating G1, and the interference pattern G2 in accordance with g0/g2=1/d, correct superposition of the wave antinodes and the wave nodes of the standing wave field is possible at least in terms of intensity. In the abbreviated formulation of the patent application, the term “quasi-coherent radiation” or “quasi-coherent radiation source” is used in this context.
The temporal or longitudinal coherence of the radiation is associated with the monochromaticity of the x-radiation or the x-radiation source. The x-radiation of intense characteristic lines usually has a sufficient monochromaticity or temporal coherence length for the applications discussed here. Upstream monochromators or selection of the resonant energy via the web height of the phase grating can also filter out a sufficiently narrow spectral range from a Bremsstrahlung spectrum or synchrotron spectrum, and thus satisfy the requirements for the temporal coherence length in the present arrangements.